TEETH. 



903 



jig. 546. B, their form mfig. 546. A; the last 

 molar, when it comes into use, presents a 

 bilobed form of grinding surface, as is shown 

 at b,fig. 575. 



Owing to there being but one set of molars 

 in the Dugong, those teeth cannot be divided 

 into true and false molars, any more than in 

 the Sloths or Armadillos. In the true Di- 

 phyodonts, in which each kind of teeth have 

 deciduous predecessors, those grinders which 

 succeed the deciduous ones vertically, and 

 displace them, are called "premolars," or 

 " false molars," and those that come into 

 place behind these, without pushing out 

 vertically any predecessors, are the " molars 

 proper," or " true molars." In this article, 

 as in my " Odontography," the two sorts of 

 grinders are called respectively " premolars " 

 and " molars." In the Marsupial order the 



normal number of molars is four in each 



4, 4, 



dental series, i. e. m. - - ; in the placental 



4 4 



Diphyodonts their normal number is three, 



3 3 



i. e. m. ; the normal number of pre- 



3 3 



molars in the Marsupialia is r , but in the 



O tj 



^ 4, 



Placentalia, it is - - : in both the numerical 



44 



character of the canines is one, i. e. 



3 3 



1 I 

 11 



; that 



of the incisors three, i. e. * r . As regards 



o ~~o 



the latter teeth, however, the number of 

 exceptions in the Marsupialia is considerable, 

 and the incisors are sometimes in excess ; 

 whilst in the placental Diphyodonts, the in- 

 cisors never exceed the typical number, but 

 frequently depart from it by suppression or 

 arrest of development. 



In fishes and reptiles, certain teeth might be 

 called " incisive," " laniary," or " molar" teeth, 

 in reference to the special adaptation of their 

 form for cutting, tearing, or bruising; but such 

 terms, in the cold-blooded classes, imply no- 

 thing more than those modifications of form ; 

 they are not significative of constant and well- 

 defined groups of teeth, and could not become 

 the names of definite parts or organs determin- 

 able and traceable from one species to another. 

 In the Mammalian orders, with two sets of 

 teeth, these organs acquire fixed individual 

 characters, receive special denominations, and 

 can be determined from species to species. 

 This individualisation of the teeth is emi- 

 nently significative of the high grade of organ- 

 isation of the animals manifesting it ; espe- 

 cially when we consider the great proportion 

 of mineral substance which enters into the 

 composition of those parts ; in the number 

 and nature of which the principle of vegetative 

 repetition, and the power of the general 

 polarising forces, have been most controlled 

 in the Mammalia. 



Originally, indeed, the name " incisors," 

 "laniaries" or "canines," and "molars" 

 were given to the teeth, in Man and certain 

 mammals, as in reptiles, in reference merely to 



the shape and offices so indicated ; but they 

 are now used as arbitrary signs, in a more fixed 

 and determinate sense. In some Carnivora, 

 e.g., the front teeth have broad tuberculate 

 summits, adapted for nipping and bruising, 

 while the principal back teeth are shaped for 

 cutting, and work upon each other like the 

 blades of scissors. The front teeth in the 

 elephant project from the upper jaw, in the 

 form, size, and direction of long pointed 

 horns. In short, shape and size are the least 

 constant of dental characters in the Mam- 

 malia; and the homologous teeth are deter- 

 mined, like other parts, by their relative 

 position, by their connections, and by their 

 development. 



Those teeth which are implanted in the 

 premaxillary bones, and in the corresponding 

 part of the lower jaw, are called " incisors," 

 whatever be their shape or size. The tooth in 

 the maxillary bone, which is situated at, or 

 near to, the suture with the premaxillary, is 

 the " canine," as is also that tooth in the 

 lower jaw which, in opposing it, passes in 

 front of its crown when the mouth is closed. 

 The other teeth of the first set are the " de- 

 ciduous molars ; " the teeth which displace 

 and succeed them vertically are the " pre- 

 molars;" the more posterior teeth, which are 

 not displaced by vertical successors, are the 

 " molars " properly so called. 



When the premolars and the molars are 

 below their typical number, the absent teeth 

 are missing from the fore-part of the pre- 

 molar series and from the back part of the 

 molar series. The most constant teeth are 

 the fourth premolar and the first true molar ; 

 and, these being known by their order and 

 mode of development, the homologies of the 

 remaining molars and premolars are deter- 

 mined by counting the molars from before 

 backwards, e.g. "one," " two," "three;" and 

 the premolars from behind forwards, e. g. 

 " four," " three," " two," " one." The incisors 

 are counted from the median line, commonly 

 the foremost part of both upper and lower 

 jaws, outwards and backwards. The first in- 

 cisor of the right side is the homotype, trans- 

 versely, of the contiguous incisor of the left 

 side in the same jaw, and, vertically, of its 

 opposing tooth in the opposite jaw ; and so 

 with regard to the canines, premolars, and 

 molars ; just as the right arm is the homo- 

 type of the left arm in its own segment, and 

 also of the right leg of a succeeding segment. 

 It suffices, therefore, to reckon and name the 

 teeth of one side of either jaw in a species 

 with the typical number and kinds of teeth ; 

 e.g. the first, second, and third incisors, 

 the first, second, third, and fourth premolars, 

 the first, second, and third molars; and of 

 one side of both jaws in any case. 



The homologous teeth being thus deter- 

 minable, they may be severally signified by a 

 symbol as well as by a name. The incisors, 

 e.g., by their initial letter i., and indivi- 

 dually by an added number, i. 1, i. 2, and 

 i. 3 ; the canines by the letter c. ; the pre- 

 molars by the letter p. ; and the molars by 



3 M 4. 



